Roh instructed his security ministers to prepare for a deployment on Tuesday, said presidential spokesman Yoon Tae-Young on Thursday.
The United States had reportedly asked South Korea for more than 5000 soldiers.
Roh agreed on 18 October to a US request to send troops to Iraq, but until now has declined to put a figure on the deployment.
Officials said the additional troops would include combat and non-combat forces without disclosing the proportion of each.
The announcement came a day after at least 18 Italian troops were killed in the southern Iraqi city of al-Nasiriya when a lorry carrying explosives slammed into their base.
The decision was also announced three days before a visit there by US Defence Secretary Donald Rumsfeld, on his first trip to South Korea since his appointment as defence secretary in 2001.
Weak international support
Earlier this week, Rumsfeld called on the international community to supply "a lot of troops" for Iraq, rocked by increasingly active resistance to US-led occupation forces.
Roh's decision came just hours after Seoul military chiefs ordered 464 non-combatants from South Korea already in Iraq to be confined to their bases, following the al-Nasiriya strike.
The South Korean army engineers and medics are based in al-Nasiriya where the attack occurred.
“They have been ordered not to engage in any activities outside military camps there. The ban will last till we can judge they are safe in carrying out work outside,” said Joint Chiefs of Staff spokesman Lee In-Young.
The US request for additional South Korean troops had split public opinion and triggered pro- and anti-troop dispatch demonstrations in Seoul.
Roh recently spoke of his personal "agony" in considering the request and had sent two fact-finding missions to Iraq to survey the risks South Korean troops would face.